Electrolytic deposition of chromium



May 29, 1945. C. G. HARFORD 2,377,229

ELECTROLYTIC DEPOSITION OF CHROMIUM Original Filed July 3, 1937 /f fasc 2r Vf ma c@ /wcu im@ @w15 5c 5f ,rf /9 2a 2/ 22 27124 2f 262; 2a 29 3013"/ 52 .75345556 i l I l l i Pas/nn? ms ,vaar/yf @as my, s

Patented May 29, 1945 Charles G; Harford, Quincy, Mass.. assigner to Arthur D. Little, Inc., Cambridge, Mass., a corporation ot Massachusetts y Original application July 3, 1937, Serial No.

151,854, now Patent No. 2,355,010, dated August 8, 1944. Divided and this application November 6, 1943, Serial No. 509,199

6 Claims.

This invention relates to an improved method of electrolysis, including the electro-deposition of metals, particularly chromium, and to the composition of the electrolyte. This application is a continuation-in-part of application Serial No. 21,809, filed May 16, 1935, and a division of copending application Serial No. 151,854, led July 3 1937, now Patent No. 2,355,070 of August 8, 1 44.

In the procedures heretofore employed in the electroplating art, it has been1` found advantageous for various reasons to prepare an electrolyte in which the metal to be deposited is in the form of a complex compound. As a result of extended experience and experiment, cyanides have been found especially suitable and are lused in large amounts in elctrolytic baths, and many technical practices of electrolysis and electroplating depend upon them. Such practices have been adopted and adhered to, in spite of the well-known intensely poisonous character of cyanides and cyanide solutions and the danger which is consequently incurred throughout the plants in which such compounds must be stored, handled, operated with and replenished from time to time.

Other procedures have been followed, it is true, but these, in general, entail the use of two or more solutions operating as a batch process. Operating in this way it is necessary to add chemicals to certain parts of the system and to withdraw and discard worthless by-products from others. In thus destroying or converting the components of the electrolyte to some other form, these processes of the prior art involve the unnecessary and hence wasteful consumption of reagents and of electric current or energy, as well.

It is accordingly an object of thisinvention to provide an improved method of electro-deposition in which satisfactory or improved results may be accomplished and in which compounds other than the cyanides, and which are essentially nonpoisonous, may be used.

Another object is to provide a method in which the current eiiiciency may be improved. 'A further object is to obtain a brighter plate.

It is also an object to provide an electrolytic process and an electrolytic medium wherein the reaction consists in effecting electrolytic transfer of the metal (anode) to metal (cathode) without side reactions or at least without side reactions resulting in the cumulative formation of ancillary by-products, and in which the only cumulative product is that of the electrolytically deposited metal on the cathode, and in which the such metal in the system, as a. metal at the anode. Economy of reagents and of electric current, accordingly are objects of the invention. Other objects will appear from the following disclosure.

A salient feature of the present invention is the discovery that chromium is subject to a uniform electrolysis and undergoes a dependable dissolution and regular deposition of the metal under the` influence of an electric current, if the electrolyte is of substantially uniform composition between the anode and the catode and contains a hydrocarbon polyamine such as ethylene diamine. It may be further enhanced if an excess of the hydrocarbon polyamine is present. The hydrocarbon polyamine added, whether in excess or not, does not appear to be permanently affected by the electroylsis and even after prolonged use of the bath it is found upon analysis to remain undiminished in its total content.

The hydrocarbon polyamine combines with the metal or with the cation of the electrolyte and particularly with the metal component of the salt, to form therewith a complexion which is readily susceptible to ionization, electroyltic transfer to the cathode, and discharge of the metal ion and deposition of the metal in solid form thereon. The metal content of the electrolyte is replenished by the direct corrosion and dissolution of the anode.

By the expression electrolytic deposition as used in the present application and in the claims, is to be understood the electrolysis of an electrolyte, containing one or more metals in solution, and forming a continuous liquid medium between the cathode and a soluble metal anode, which includes the separation and deposition of said dissolved metal or metals upon the cathode,

and simultaneously dissolving of the metal anode.

, thereby replenlshing and maintaining the metal content of the electrolyte.

The anodes, formed chromium are uniformly and regularly dissolved into the electrolyte without disintegration and the resulting formation of residual granules or powder.

The process is characterized by being applicable to metals which are able to form complex cations containing a hydrocarbon polyamine. The proof that such' complex ions are formed is that the metals in the form of the complex ion can exist' in alkaline solution whereas the corresponding simple metal ions are precipitated when made alkaline, as by-the hydroxide of an alkaline metal.

Hydrocarbon polyamines contain carbon, nitrogen, and hydrogen and no other elements. They only required addition is that of replenishing 56 are thus distinguished from ammonia, and also from the alkylolamines which contain oxygen, both of which are unsuitable for use in the present process. Hydrocarbon polyamines suitable for use in the present invention contain at least two atoms of nitrogen, each of which is joined to a carbon atom but not to a nitrogen atom; they are water-soluble and denitely basic. Examples of suitable hydrocarbon polyamines are ethylene diamine, propylene diamine, and the polyethylene amines. The latter are considered to be condensation products of ethylene diamine with itself or with other aliphatic amines; one of the simplest of such products would be:

HzN-CHz--CHa-NH-CHa-CHa-NH:

'I'he general formula of hydrocarbon polyamines suitable for use in connection with this process is therefore:

where nis 2 or more and R is hydrogen or the radical (CHnn-NHR R being the same as just explained. The process of the invention, as described in the above mentioned applications, may be carried out, for example, with salts of certain monovalent metals, such as silver, with salts of bivalent metals, such as copper, cadmium. zinc, and nickel. with salts of trivalent metals. such as iron and chromium, and salts of tetravalent or hexavalent metals, such as platinum and tungsten. in the presence of the polyamine. In these metallic polyamine complexes, the polyamine and the metal are attached to each other by the secondary valences of the nitrogen and of the metallic atoms. Since only secondary valences are involved, the entrance of the polyamine into the cation does not alter the chargeur valence of the cation.

The metals with which the invention may be carried out, as described in the above mentioned applications, are indicated in the chart of Fig. 1,

in which the symbols of the elements are shown in the order of their atomic numbers, arranged in accordance with the periodic table. The metals with which the present invention, as described in the above mentioned applications, is applicable are enclosed by the dotted line.

Fig. 2 is a more or less diagrammatic illustration of the type of electrolytic cell and indicative of the type of electrolysis with which the process may be practiced, namely, one in which the elec-- trolyte is continous from electrode to electrode.

In practice, it is now found that when chromium is made the anode, with respect to an elec-- trcally conductive cathode, and an aqueous fluid electrolyte is provided, extending from the surface of one to the surface of the other without interruption, containing a soluble hydrocarbon diamine as above defined, the application of an electrica1 potential or current between the electrodes will result in the dissolution of the anode and the electrolytic disposition of the metal upon the cathode, without side reactions and without change of regulation other than the addition of metal as metal to the anode and removal of the deposited metal on the cathode.

It is found that a water solution of the amine alone is suicient for such operation, the metal from the anode dissolving and forming complex cations in the electrolyte and undergoing electrolytic deposition without the addition of any salt of the metal and without any indication that salt formation of the metal occurs. On the other hand, a salt of the metal or metals involved may be present to advantage, especially where it is economical to employ.

In either case, however, it is essential that the uid medium of the electrolyte between the anode and cathode be unobstructed, continuous, and preferably uniform, as by agitation.

In Fig. 2 it will be observedtthat the electrolytic solution I is contained in a single vessel 2, which is preferably lined with rubber, and forms a continuous medium between the anode or anodes 3 (which are connected to the positive bus bars 4, and are composed of the metal or metals to be deposited) and the cathode or cathodes 5, which are connected to the negative bus bar 6, and upon which the metal is to be deposited. The uniform-- ity of the electrolytic solution during operation may be promoted-by agitating the electrolyte with any suitable means (not shown) but is substantially automatically preserved throughout the reaction which is accurately represented, in its net eiective result, by the equation Metal anode-Metal (cathode) for there is no cumulative consumption or conversion of the other constituents of the electrolyte into by-products and hence no necessity for replacement or reconversion, even upon long and continuous operation.

It is found that hydrocarbon polyamines react with ionized salts of chromium, in aqueous solution, to form various orders of compounds. The number of compounds so formed varies, in accordance with the number of secondary valences i as set forth in theories of Werner complexes)` but ned not be gone into here in any great detail as the invention will be clear from the disclosure herein.

It is generally preferable to use at least 2 mols of hydrocarbon polyamines to one mol of the salt of chr ium, and best results range between about 21/2 and 6 mols of the polyamine to one of the salt. More polyamine may bc used, but there is noparticular advantage in so doing.

Typical and representative examples of the` practical application of this invention will now be described in its relation to electroplating, with respect to chromium and to various salts of the metal.

In the examples, the ethylene diamine is given on the anhydrous basis, although it is customarily available in the form of a 40% or 60% solution in water.

In carrying out the process of this invention. the temperature of the bath is preferably at, or somewhat above, room temperature, generally between 25 C. and 40 C.

The electrolyte I may be contained in any suitable resistant vessel 2 which is preferably a non-conductor of the electric current, such as earthenware or rubber. The anode 3 (or anodes) may conveniently consist of chromium strips which are connected to a source of electric current and are suspended in the solution, preferably so as to be completely submerged, as by bus bars 4, having a chemically resistant or protected conductor. The cathodes-were connected to a suitable source of electric current to provide a not on account of any failure or decline in operation) the amount oi' ethylene diamine remaining in the electrolyte was substantially undlminished.

While ethylene diamine is set forth in the ex` ample as the hydrocarbon polyamine used, it may ordinarily available commercially, but it may be converted to the violet form by heating with ammonium chloride'(foi;4 example in the ratio of 2 to 1, respectively) to about 600 F. until no more vapors of ammonium chloride come off. -The bath is prepared as follows:

Chromium chloride as prepared above grams" 100 Ethylene diamine do 500 Acetic acid To make acid to litmus Water M' 1000 sing metallic chromium for the anode material, electrolysis is carried out in the manner already described, and a bright coat of chromium is obtained on the cathode, which may be any 1 suitable metal such as copper or brass. Temperature of the bath is adavntageously around .40 C. As indicated, the bath should be somewhat on the acid side of neutrality that is, manitesting a pH below 7.0.

It is possible to use more than one salt of the same metal in the plating bath, if desired.

It is also possible to plate alloys by the present process. For example, brass may be successfully plated by using brass anodes and a bath of copper and zinc salts.

As described in the above mentioned application, itis possible to carry out the process of this invention without first dissolving a salt of the metal to be plated in the bath. In such cases,

j the metal of the anode dissolves to furnish suiiicient metallic lons in the bath, and plating proceeds. Such 'procedure has no particular advantage when plating with most ordinary metals, but when the rarer metals are used, such as `tungsten and platinum, and metals of the platinum group, it is generally inconvenient and expensive to use their salts in the bath, and adequate results can be obtained as far as such metals are concerned when an anode of the metal to be plated is placed in a bath containing a hydrocarbon polyamine and the plating carried out as described above. For instance, a tungsten anode is immersed in a bath containing 25% ethylene diamine in water.

Platinum and other metals of the platinum group may be plated in the same way. Alternatively, if desired, salts oi.' these metals may be added to the plating baths and the procedures of the preceding examples may be used.

The cathode in operating with the solutions described herein, maybe substantially any clean, smooth, electrically conductive metallic surface.

The operation of the reaction is smooth and con-l tinuous, forms a bright, uniform deposit of metal on the cathode, and may be prolonged for substantially any desired period of time without care or control, other than the supply of an electric current, and replenishment of the metal of the anodes, and removal and replacement of, the

plated cathode, from time to time, if and when.

desired.

If' there is any tendency for basic salts of the electrolyte to separate, as for example, at the cathode (with solutions containing a, lower ratio of hydrocarbon polyamine to'metallic salt than 2:1), which might interfere with the continuity of vthe deposition orof the plated surface coating, this may be overcome by the addition of sulfuric acid (in case of sulfate salts or corresponding acid in case of other salts) or of the polyamine, or both. Agitation of the bath or of the cathode is likewise helpful.

I claim:

1. An aqueous electrolyte for the electrolytic deposition of chromium characterized by consisting essentially of a soluble salt of chromium and an alkyl hydrocarbon polyamine and maniesting a pH value below 7.0.

2. An aqueous electrolyte for the electroiytic y containing an aqueous electrolyte consisting es-.

sentially of a soluble salt of chromium and ethylene diamine and manifesting a pH value below '7.0.

5. An aqueous electrolyte for the electrolytic deposition of chromium, characterized by consisting essentially of a soluble salt of chromium andan alkyl hydrocarbon diamine and manifesting a pH value below 7.0.

. 6. A process of electrolysis that comprises elec` trodepositing chromium from an undivided cell containing an aqueous electrolyte consisting essentially of a soluble salt of chromium and an alkyl hydrocarbon diamine, and manifesting a pH value below '1.0.

GHARLES G. HARFORD. 

